(Meth)acrylic resins have long been widely used in various fields as glass substitutes for their transparency, luster, surface hardness, weather resistance, and mechanical properties. For example, (meth)acrylic resins are used as capstocks covering the surfaces of materials for outdoor use, display panels of mobile phones and the like, and transparent members for display housings. Further, (meth)acrylic resins are used in optical devices such as lenses and prisms, and are recently used as substrates for information recording media such as video discs, audio discs, information discs, and optical cards and optical device materials such as optical fibers for their excellent moldability and low birefringence. The presence of impurities in an optical device such as an information recording medium substrate causes errors in, for example, reading of recorded information signals, which reduces the reliability of the optical device. The presence of impurities in a plastic optical fiber causes absorption or interruption of light so that optical transmission loss is increased, which reduces the performance of the plastic optical fiber. For this reason, (meth)acrylic resins are required to have less impurities when used as optical device materials.
(Meth)acrylic resins are produced by radical polymerization such as bulk polymerization, suspension polymerization, or emulsion polymerization. Particularly, bulk polymerization is most commonly used for the reason that (meth)acrylic resins having less impurities can be obtained. In the case of bulk polymerization, a (meth)acrylic resin product finally obtained is in the form of pellets. However, since many of materials to be added for the preparation of a composition, such as an impact resistance-improving agent and a processability-improving agent, are in the form of powder, there is a case where the pellets and the powder are separated from each other during compounding, transfer, and extrusion molding so that the components of the composition are non-uniformly mixed or dispersed.
On the other hand, in the case of suspension polymerization, particles having a diameter of several tens of microns to several thousand microns can be obtained by polymerization, and therefore a (meth)acrylic resin product obtained through the steps of washing and drying is a spherical powder, which is advantageous in that excellent mixing/dispersion performance is achieved when the (meth)acrylic resin product is compounded with another powder material. However, a suspension stabilizer and a suspension aid need to be added in large amounts as auxiliary materials to maintain stable dispersion of monomer-polymer droplets during polymerization. Therefore, it is pointed out that a small amount of impurities remain in the product even after washing, which becomes a cause of thermal degradation during molding processing. If the amount of the suspension stabilizer used is reduced, the dispersion stability of monomer-polymer droplets is reduced due to a reduction in the concentration of the suspension stabilizer, and therefore there is a case where the coalescence of monomer-polymer droplets occurs so that polymer particles having a wide particle size distribution are produced or agglomeration occurs in the whole of a polymerization system when conditions are unbalanced. For this reason, the problem of thermal degradation during molding processing (more specifically, deterioration in transparency and yellowing during molding processing) makes it more difficult to select the type of suspension stabilizer used and to adjust the concentration of the suspension stabilizer in terms of stable production and particle size control.
Under the circumstances, Patent Document 1 discloses, as a method for producing a high-purity (meth)acrylic resin by suspension polymerization, a method in which a mixed solution of water and an organic solvent having compatibility with water is added to a particulate polymer obtained by suspension polymerization to extract and separate a low-molecular-weight organic compound or a water-soluble inorganic compound from the polymer. In this method, an aftertreatment step is additionally performed after suspension polymerization, but addition of such a step itself not only complicates production but also has the possibility of introducing other impurities.
Patent Document 2 discloses a method in which suspension polymerization is performed in a polymerization tank under a specific stirring condition to produce thermoplastic polymer particles having less impurities. However, in this method, introduction of impurities into polymer particles is prevented only by controlling mechanical factors, and therefore it cannot be said that introduction of impurities is satisfactorily prevented, and in addition, the selectable range of the stirring condition is limited.
Patent Document 3 discloses a method for producing a (meth)acrylic resin for optical discs by suspension polymerization using a surfactant and sodium sulfate or sodium phosphate, and states that a superior optical disc substrate having less microbubbles can be obtained when the amount of cations remaining in the resin is smaller. In Patent Document 3, evaluation of impurities is made based on the amount of Na ions contained in the resin, but there is no specific description about the surfactant. For this reason, this method cannot be appreciated as a resin production method.
Patent Document 4 discloses a method for producing a high-purity (meth)acrylic resin having less impurities by suspension polymerization of a monomer mainly containing methyl methacrylate in an aqueous medium, in which an anionic water-soluble polymer is used as a suspension stabilizer at the start of polymerization and a nonionic water-soluble polymer is further added when the degree of polymerization is 20 to 85% to continue polymerization. However, in this method, polymerization is initiated in the presence of a relatively large amount of suspension stabilizer composed of an anionic water-soluble polymer, and therefore the anionic water-soluble polymer as a suspension stabilizer remains in a resulting particulate polymer, that is, only a low-purity particulate polymer is obtained. Further, this method still involves a problem such as deterioration in transparency or yellowing during molding processing.
As described above, it is difficult to obtain, by suspension polymerization, a (meth)acrylic resin that has less impurities and is less likely to deteriorate in transparency or turn yellow by thermal degradation during molding processing.
Patent Document 1: JP-A-62-209108
Patent Document 2: JP-A-56-8404
Patent Document 3: JP-A-1-231001
Patent Document 4: Japanese Patent No. 3118877